Cabbage worms can do that. They like brassicas. Bt will get rid of them, but Sevin should do so as well. It's not pretty, but your damage doesn't look crippling.

While the greens are edible, most focus on the root itself so at this time of the season, any leaf damage is pretty inconsequential. Since you must be getting close to harvest time anyway, I see no reason to treat with anything now but I'd plan on some insect protection next season.

Well, depends on where you are. Down here we often do an early fall planting for harvest in the spring. Like for turnips, it's too hot for good summer crops. You get bitter, tough roots if you try to grow them through summer here. But that's true, if you're close to harvest, don't worry about it.

The plants look quite small still, so I imagine harvest is some way off. But I still wouldn't worry about some foliage damage going into autumn. Next year cover them with fleece or fine netting to exclude the butterflies.

My bet is womrs like Dan said, BT it (organic) look under the leafs I be you see them they tend to mtacht the leaf color really good.